Ceramic body



J. W. NORTH CERAMIC BODY Filed June 4, 1957 Oct. 23, 1962 INVENTORC JOHNW NORTH AT TORNE Y United States Patent 3,059,296 CERAMIC BODY JohnWalton North, Decatur, Ga., assignor, by mesne assignments, to GlasrockProducts, Inc., Atlanta, Ga., a corporation of Georgia Filed June 4,1957, Ser. No. 663,398 2 Claims. (Cl. 22-493) This invention relates toceramic body and more particularly the present invention relates to areusable foundry mold formed essentially of amorphous silica and to themethod of preparing and forming the mold and thereafter repairing themold as required by use.

In the prior art there are several types of molds which may be used forcasting molten metal into predetermined shapes. A sand mold is one suchtype and is considered a temporary mold since it may only be used onceto provide one casting. In such a mold, the sand is bound by a liquid ormoistened bonding agent into a bulk of predetermined shape and may beleft in its moistened condition or dried. If the sand contains water andis in a moist condition when the molten metal is poured into the mold,the water will be vaporized and must escape through the body of themold. Therefore, the sand must be sufficiently coarse or loose thatventing of the vapor or gas through the sand will be permitted. If otherliquids are employed the mold is usually more expensive and noxiousfumes may be produced when the molten metal is poured. On the otherhand, drying of the bulky sand mass of the mold is time consuming andexpensive and there is always a danger of distorting the casting cavityduring drying.

' Another problem in the prior art use of the green sand mold is thefact that it must be bulky and heavy in order to withstand hydrostaticpressures exerted by the molten metal without undue alteration of thedimensions of the casting cavity. Thermal expansion of the green sandcharge, when the molten metal is received is another ever presentproblem. Because of these difiiculties, the ramming pressures of themold are controlled within practical limits in the foundry; however,even with extreme care, green sand castings always have some variationsin surface finish and dimensions, the result. being that, in manyinstances, the castings have excessive weight, require excessivemachining or must be rejected.

More recently, a molding system termed shell molding has been employed.The shell mold is usually prepared by depositing a quantity of sand andresin in a heated flask whereby the resin bonds the sand in a shellsegment which, when joined with other segments, defines the castingcavity. A shell so formed is used only once and usually gives offnoxious fumes when the resin burns or is polymerized. The cost of theresin usually makes a shell mold expensive and the cost of the equipmentemployed to form the shell prevents small scale use of this system. Thecastings from shell molds are usually superior in surface texture togreen sand; however, surface defects and dimensional deviations arepresent to a great extent therein.

Die casting, investment molding, and other precision casting systemshave also been employed. Thesesystems usually require a substantialamount of complicated machinery and, in some instances, expensivepermanent molds. Such systems sometimes employ fine grained materials onthe surfaces of the mold cavity to impart a smooth surface to thecasting. ln such molds, a colloidal silica binder and also a crystallineform of silica which has a more favorable temperature expansioncharacteristic than quartz sand, have each been used as surfacingagents. With thermal expansion and contraction of the V mold, however,cracks in, and flaking of, the silica surice face occur, requiring suchsurfaces to be replaced sub stantially each time the mold is used.

In addition to being expensive, the prior art permanent molds are noteasily adapted to relatively complicated shapes requiring a crushabletype cavity area whereby a portion of the mold cavity must be removedwith each casting and that portion replaced for a successive casting.For this reason and others, precision methods are not generally employedfor large or complicated castings.

Permanent dies usually are made of metal and therefore are limited bythe melting point and reactive characteristics of the metal within themold. These dies, as pointed out above, are also expensive.

The patent literature discloses other types of molds such as thosefor-med of various types of crystalline silica such as silica flour. Itis believed that such molds would have expansion and thermal shockcharacteristics which would cause the mold to crack or deteriorate withrepeated use since the thermal expansion of silica flour is about tentimes that of the material of the present invention, namely amorphoussilica.

It is generally accepted in the art that amorphous silica is unstableover about 1000 C. and therefore would be unsuitable for use as areusable mold body since the temperature of the molten metal which themold body receives is usually in excess of this critical temperature,the assumption being that the mold body would be heated by the moltenmetal to a temperature in excess of the critical temperature. Textbook-s vary as to the exact temperature at which the amorphous silicawill become unstable; however, the unstable eondition which is attainedby the amorphous silica is called devitrification, :and is characterizedby the amorphous silica converging into cristobolite. One text bookstates that at 870C. =B- quartz changes suddenly to Bg-tridymite, and at1470? C. this changes to B-cristobolite. At 1625 C. cristobolite meltsto silica glass. On cooling, B-cristobolite changes at temperaturesranging from 222 to 275 C. to another form called X-cristobolite. Othertext books state that the main disadvantage of silica as a refractorymaterial is its tendency to spall and crack during heating and cooling.For these reasons, heretofore, no reusable permanent type mold for useabove a temperature of 1000 C, has been devised utilizing essentiallythe amorphoussilica,

Contrary to the prior art beliefs and practices, 1 have devised aceramic body which, for convenience, may be discussed as a mold formessentially of silica in fine granular or flour for-m which may beutilized for receiving molten metals whose pouring temperatures arerelatively high, my mold being reusable for successive castings andreparable, as will be brought out more fully hereinafter. The moldformed according to my invention is highly resistant to thermal shockand thermal distortion, thereby permitting, in some instances, the.forming of simpler surfaces within the casting and the forming of gatingand riser systems directly in the mold body.

It is also contemplated by my invention that more complicated molds maybe formed by employing the shaped bonded amorphous silica for the mainbody of the mold and a shell or cope of bonded sand within the castingcavity or defining a portion of the casting cavity. In the latterinstance the sprue, ingate and riser systems are preferably defined bythe main mold body of my invention. 'By such an arrangement, the shellor bonded sand portion of my mold may have weaker bondingthan hasheretofore been deemed advisible, without sacrificing dimensionalaccuracy, since the shell portion of my mold is backed by the main moldbody.

When molds are prepared, according to my invention, the amorphous silicamay be recovered after the mold has served its useful life and thismaterial reprocessed r 3 for use in forming other molds. It is alsoapparent that the amount of expendable material employed, in forming theshell or core is reduced to a minimum.

No sacrifice in quality of the resulting casting is necessary toaccomplish economical operation, according to my invention, since thecasting therefrom would be considered superior to a casting from green'sand and would, in many instances, be comparable to a casting frominvestment molding. I have found the castings from my mold to be withinclose dimensional tolerances and essentially free from distortion, withthe surfaces of the castings being relatively smooth. 7

Accordingly, it is a primary object of my invention to provide aninexpensive permanent or reuseable mold and a process of manufacturingthis mold.

Another main object of my invention is to provide a reuseable moldformed of amorphous silica so disposed that molten metals having apouring temperature above the critical temperature of amorphous silicamay be poured into the mold without appreciably damaging the structureof the amorphous silica.

Another object of my invention is to provide a reu'seable mold which isresistant to thermal shock and distortion.

Another object of my invention is to provide a reuseable mold which maybe easily and effectively repaired, if necessary.

Another object of my invention is to provide a reuseable mold formed ofa material which, when the mold has served its useful life, the materialtherein may be reprocessed for use in other molds made according to myinvention.

Another object of my invention is to provide a reuseable mold, part orthe entire area of which may be easily cooled or heated by externalmeans while the mold contains the casting metal.

Another object of my invention is to provide a lightweight reuseablemold of ceramic material which is suitable for retaining casting metalunder pressure.

Another object of my invention is to provide a process of repairing aceramic mold.

Another 'object of my invention is to provide a means and method ofinexpensively and effectively producing a superior type of metalcasting.

Other and further objects of my invention will become apparent from thefollowing description 'when taken in conjunction with the accompanyingdrawings wherein like characters of reference designate correspondingparts throughout the several views and wherein:

FIG. 1 is a plan view "of a cope box or flask adapted to receive ceramicmaterial to form a segment of a mold construc'ted'in accordance with myinvention;

FIG. 2 is 'a cross sectional view taken along line 22 in FIG. 1, thecope box being shown containing damp ceramic material;

FIG. 3 is a cross sectional view similar to the view of FIG. 2 showingwet ceramic material after it has been standing in the cope box;

FIG. '4 is a cross sectional view of another flask or dragbox providedwith damp ceramic material for forming the other segment 'of my mold;

FIG. 5 'is'a cross sectional view similar to the view shownin FIG. 4showing wet ceramic material after standing in the drag box;

FIG. 6 is a plan view of an assembled mold constructed of the moldsegments shownin FIGS. 3 and 5; and

FIG. 7 is a cross sectional view of amodified form of my inventionshowing an assembled mold having a bonded sand partial shell.

In more detail, the ceramic body of my mold segments, by way ofexamples, may be formed following either of two methods which I shallterm respectively the damp aggregate method and the wet aggregatemethod. In either method Iemploy silica aggregates formed essentially ofgranulated or powdered amorphous silica, fused silica or high silicaglass. Preferably, as pure a silica as possible should be used for bestresults; however, I do not wish to exclude the use of a silica material,such as high silica glass which contains above amorphous silica, whichwould give satisfactory results, according to my invention.

The silica aggregates should be sufficiently fine in size that they willpass through a 30 mesh screen, since larger particles with no fin'erparticles intermixed therewith do not band together as readily. Iprefer, however, that the aggregates be sufiiciently fine to passthrough a mesh screen since such aggregates form a smooth surface andband together well. It is not necessary that all particles of the silicaaggregate be uniform in size. Amorphous silica or high silica glass,which is ground to a size on the order of silica sands, crushed sands,or flours, is suitable for use. The amorphous silica or fused silica,which is a more limited term for amorphous silica, diliers from sandcommonly found in nature, in that the amorphous material has been fused,then cooled quickly, then comminuted to a small size. Of course, othermethods may be employed for obtaining the amorphous silica such asfusing the silica granules as they fall freely and then cooling thegranules by permitting them to continue to fall. Round shots are formedin this manner and these round shots are suitable for use as my silicaaggregate.

In either method of preparing the mold body, I bond the silicaaggregates with a liquid suspension of colloidal silica which, whenmixed with the silica aggregate, may be caused to set, as will bedescribed hereinafter, to form a monolithic body. Being amorphous silicaitself, the silica of the binder has essentially the samecharacteristics as the silica aggregates and therefore is equally ableto resist thermal shock.

In making the mold body according to the damp aggregate method, thebinder is prepared by suspending from about 2% to about 35% colloidalsilica by volume with water or some other suitable vehicle such asalcohol. The manufacture of a colloidal silica suspension is disclosedin the prior art and therefore no detailed discussion of its productionis necessary. Such suspensions are available commercially under tradenames such as Ludox produced by the Grasselli Chemical Co., Inc., ofWilmington, Delaware, department of E. I. Du Pont de Nemour; and Nalcoagproduced by National Aluminate Corporation of Chicago, Illinois. Thesecommercially available colloidal dispersions are suitable for useaccording to my invention in the form in which they are shipped.

The damp aggregate is prepared by mixing or commingling about 100 partsof the silica aggregate with from about 4 to about 15 parts of binderprepared as above described or as commercially received. 7 If thischarge of silica aggregate and binder is not to be used immediately, itshould be kept covered so that it does not dry out.

When forming a mold segment, the charge is shaped to define a portion ofthe casting cavity together with the connecting sprue, gating and risersystems in a-manner similar to forming a green sand mold. In FIGS. 1 and2 I have disclosed one convenient way to form a mold segment, wherein acavity pattern 10 is mounted centrally on a pattern plate 11 havingupstanding sides 12 and ends '13. The sides 12 and ends 13 together withthe plate 11 define a flask or cope box for receiving the damp chargedenoted by numeral 14. In the present embodiment, the sprue pattern 15,ingate pattern 16, riser pattern 17- and plenum chamber pattern 18 aredis posed with cavity pattern 10 to define a conventional patternarrangement. 7

As is well understood in the art, it may be necessary to prepare thevarious patterns and the pattern plate for easy removal from the moldsegment 19 formed by Ail charge 14 by treating these parts of the copebox with a releasing agent such as wax, silicone, chlorinatedhydrocarbon and the like. Th3 use of these releasing agents is wellknown and therefore mere mention of some releasing agents is sufiicient.In cases where the patterns are formed from gypsum plaster or othermaterial which will shrink, such patterns'should be soaked in water andthen dipped, sprayed or coated with wax, grease, or lacquer, so that ifthe patterns shrink upon drying of the mold segment in the cope box, theoriginal shape of the pattern will remain in the mold segment.

Patterns for my mold may also be made of wax which may be melted fromthe mold or burned out. Ofcourse, a pattern may be made from the dampaggregate itself, in which case it should be coated as by lacquer orvarnish after final shape is obtained and the wet mix poured around it.The mold segment should then be fired, after gelling or drying, untilthe coating of lacquer or varnish is burned out. This leaves the patternin a loose con dition. i

If the pattern is made of metal, it should not be fired to hightemperature or oven heated with the mold segment since the metal patternwill expand and crack the mold segment. Other methods and techniquesused in fine metal work and ordinary foundry practice are also adaptableto my moldcasting system.

It is to be noted that if great accuracy and smooth: ness of casting isdesired, I prefer to employ a pattern made of the same aggregate mix asis to be used for the mold segment, the aggregate mix beingcoated asdescribed above and fired with the mold segment. Further, the entirecope box may be made of the charge 14 so that it will have the sameshrinkage as the mold segment 19 when dried and/or fired with the charge14.

According to my damp aggregate method, the charge, prepared as abovedescribed, is placed in the flask in a manner similar to the charging ofgreen said into a Cape box. For strength, the moldsegment 19 thus formedis then dried by removing the water from the colloidal silica to leave ahard ceramic body. For additional strength, mold segment 19 may besprinkled or asperged with additional binder, and then redried.Sintering in an oven at about 1000. F. to 1900" F. for a few minuteswill strengthen mold segment 19' even further.

It is noted here that once the binder is dry or has been sintered,rewetting does not redisperse or. cut the dried binder. .Further, themold segment may be dried, sin-. tered, and rewet in any sequence, justso that the drying is completed before the mold segment 19* is rewet.

After drying, mold segment19 is removed from the cope box and inspectedfor imperfections. If damage has occurred to mold segment :19, it may beeasily repaired by spreading additional charge with a spatula, trowel,screeding tool or other suit-able instrument over the broken area andthereafter carving, scraping or sanding the surface so formed, eitherbefore .or after it has dried. If the damage is superficial, the moldsegment 19 may be used before the repaired portion is dry. Of .course,if the damage is severe, mold segment 19, after being repaired, shouldbe redried and/ or refired.

In the same manner as the cope boxof FIGS. 1, 2 and 3, the drag boxshown in FIGS..4 and 5 includes a pattern plate 20, sides 21 and ends22. The charge is supplied to the drag box in a manner similar to thecharges of the cope boxdescribed above. The mold segment 23 thus formedis dried and .sintered as described for moldsegment 19. In thisparticular. instance, simply a flatrnold segment '23 is prepared andhence noconfiguration, other than a flat surface, is imparted tothecasting cavity area of mold segment 23 or other areas thereof. .It.willbe understood, however, that any conventional pattern may be utilizedfor a particular configuration to be imparted to.

mold segrnent 23 if desired. v I A After being prepared properly, theceramic bodies formed as the mold segments 19 and 23 are removed 6 fromtheir fiasks and joined as shown in FIG. 6, the abutting surfaces of themold segments being contiguous along a part line 24. For retaining themold segments in appropriate registry, a C-clamp 25 is provided havingabutments 26 which urge the respective mold segments together. The moldis thereafter ready to receive the molten metal via'sprue 27 into thecasting cavity 28.

In the wet aggregate method of preparing my mold, parts of the amorphoussilica aggregate are mixed and thoroughly commingled with from about 14to about 38 parts by weight of the binder. For purposes of illustration,I have employed the same cope and drag boxes as previously used. The wetaggregate charge, prepared of the amorphous silica and binder in theproportions above stated, is poured into the boxes. Usually it isdesirable to fill the boxes about half full and in all instances coverthe entire pattern with the wet charge. Thereafter, the charge ispermitted to settle. For reducing the time necessary for settling, theboxes may be vibrated in a vertical direction until the particles of thecharge are well packed and as much as possible of the liquid, denoted bynumerals 29 and 30, has been driven to the top. Jolting of the boxes maybe accomplished by electromagnetic or air-mechanical means (not shown).When the liquids 29, 30 float to the top of the boxes they should bepoured off to facilitate setting up of the wet charge.

If it is desired to set the binder, rather than to dry the charge asdescribed above, an electrolyte is added to the wet charge eitherimmediately before or after it has been dumped into the boxes. If thewet charge is to be kept for extended periods of time, of course, theelectrolyte'should not be added thereto until it is ready for use. Theelectrolyte causes the colloidal silica to gel so that the ceramic bodybecomes hard, as does the dry ing of the water from the damp aggregatecharge in the previous method. Descriptions of methods of gelling thecolloidal silica are disclosed in the E. I. du Pont de Nemours Productinformation bulletin entitled Du Pont Ludox Colloidal Silica and inother literature. Therefore, little description is necessary as toparticular methods of gelling the colloidal silica except to state thatsubstantially any electrolyte when added to the colloidal silicasuspension will tend to gel the silica. For my purposes, from .5 toabout 4% ammonium chloride, added to the colloidal silica suspension,will cause gelling'. Also, suflicient hydrochloric acid added to the colloidal silica suspension to provide a pH of from 5 to 6 will causegelling for the purposes herein described.

I prefer to use either ammonium chloride or hydrochloric acid as agelling agent or electrolyte since both of, the above chemicals may beevaporated from the charge after gelling of the col-loidalsilica andtherefore no impurities willrernain to, contaminate the silica moldsegment. gelling of the colloidal silica much faster than the chloridesmentioned above and, in the event that the amorphous silica is not to bereused, perhaps this electrolyte" would be more desirable as a gellingagent. If the gelling agent is added to the charge before the same isjolted, it would be advisable to ascertain, in

advance, the time lapse involved in gelling so as. to be certainthat thejolting is completed before gelling has proceeded noticeably. The timerequired for gelling cannot be describedhere because there are too manyvariables which would effect it. 7 Therefore, by simply taking a part ofthe particular charge and adding the gelling agent thereto before thegelling agent is added tothe entire chargeis an easymethod ofdetermining the gelling time for the particular charge. p v

"In all cases where the binder of my wet aggregate charge is to begelled, the same should be accomplished before the mold segments,denotedby numerals 19' and 23' are'dr-ied; since, if drying is permittedbefore gelling,

a portion of the binder will be drawn to the surface of Magnesiumsulfate, however, will accomplish the mold by capillary action before ithas had an opportunity to gel, thereby enriching the outer surface withbinder while making other parts of the mold segments lean in binder,whereby cracks in the mold segment may develop.

After the mold segments '19 and 23' have been hardened Within the copeand drag boxes, the pattern plates 15, are removed and pattern 10withdrawn. The mold segments 19' and 23' are then assembled as shown inFIG. 6 for the assembly of mold segments 19 and 23.

Before pouring molten meta-l into the mold, it is advisable, in order toaid in the release of the casting from the mold, to coat the areas to becontacted with a releasing agent. Soot, smoked on with an acetylenetorch, or other carbon such as graphite or carbon black may be employed.In liquid suspension, the carbon or graphite may be sprayed on. Furtherwater suspensions of silica, zircon, alumina, zirconia, calciumsilicate, magnesium carbonate or powdered aluminum may be employed.Also, various clays in combination with the above substances may beused.

Referring now to FIG. 7 of the drawing, it is seen that my silica moldsegments, denoted generally by numeral 31, are aligned by a conventionalkey 33 fitting into a key way and held in registry along a part line 24'by C-clamp 25 and abutments 26. Numeral 32 denotes a foundry sand shellor core cake which may be cast with my silica mold as shown. Many meanssuch as adhesion or dowel pins (not shown), may be used for retainingthe foundry sand shell 32 in place. The construction of FIG. 7 is usedwhen a casting is so shaped that it would be difficult to remove from amonolithic mold such as shown in FIG. 6. In preparing the composite moldof FIG. 7, the mold segments 31 are prepared with an oversize castingcavity as outlined above according to my damp aggergate or wet aggregatemethod. Thereafter, the space left in the mold for the shell 32 isfilled with a bonded sand insert or an external core print formedaccording to prior art practices. Of course after each pouring, a newinsert or shell 32 must be provided.

For cooling the shell 32, it may be found desirable to provide a venthole 34 in mold segments 31 and behind shell 32.

It will also be understood that the composite mold of FIG. 7 may beprovided with a bonded sand shell completely surrounding the castingcavity. Such a composite mold may prove desirable with extremely hightemperature pourings.

When forming a bonded shell completely surrounding the casing cavity,several vent holes such as hole 34 may be provided in the mold segments.One advantageous method, according to my invention, for forming acomplete shell around the casting cavity is to make each of the moldsegments 31 oversize and then fit each segment respectively over thepatterns, such as pattern 10. Thereafter, the bonded or foundry sand isblown into the area between the segment and the pattern through venthol'es such as hole 34. Of course, more than one vent hole 34 must beprovided so that the air or other impelling fluid may escape, leavingthe sand in place.

The mold segments are then removed from the pattern and joined together.The molten metal may then be charged into the casting cavity of thecomposite type mold while the shell is still damp or the shell may bedried to reduce to a minimum the gassing of the sand.

By the term bonded sand 'as used herein, it will be understood that Imean conventional foundry sands such as silica sand impregnated withcereals, proteins, oils, clays, sugars, resins, or sodium silicatesolution, and also coke fines, zircon, alumina, amorphous silica, aloneor in admixture with carbon or aluminum powders and/or combined With theabove binders.

To test the fact which I have discovered, that the amorphous silica, ifprepared in the form of a mold as above described, will withstandtemperatures far in excess of the critical temperature of 1000 C. Wherethe amorphous silica is expected to be converted to a crystalline form,samples of the silica mold material prepared according to my inventionwere fired at 2600 F. One sample was maintained at 2600 F. for 15minutes, another 30 minutes, another 45 minutes and still another forone hour. All samples were removed and placed in air for rapid cooling.When cooled, the surface of the samples seemed to become harder, ratherthan softer. The samples also showed good resistance to thermoshock, inthat the last sample of the material was cycled, by first heating thesample to 2600 F. and then air quenching for four cycles. No noticeablechange took place in this sample.

To further test the characteristic of my silica mold, blocks of silicawere formed according to the process outlined above. The first block was2 inches by 2 inches by /2 inch and Was placed on a supporting surfaceon its 2 inch by 2 inch side with a thermocouple extending from beneaththe supporting surface into the block to within inch of the uppersurface. This sample block was inserted into an oven which had beenpreheated to 2600 F. (1427 C.) and the time was measured for thethermocouple to indicate that the block was at oven temperature. Itrequired 5 minutes for the interior of the block to be heated to oventemperature.

Next, two samples, each a block 4 inches by 1 inch by /2 inch, wereplaced on their ends in an oven which was preheated to 2600 F. The firstblock was left in the oven for 5 minutes and then removed for cooling inair. When cool, this block was compared with an unheated similar sampleby X-ray analysis. No change in crystalline structure in this block wasobserved, thus indicating that no additional crystobolite had formed.

The second block was left in the oven 10 minutes and examined in amanner similar to the previous sample. This later sample indicated thatsome crystobolite had formed. From other test, however, it may be statedthat even if the block were left in the oven for 96 hours, no additionalappreciable change in structure would have taken place.

Therefore, since in practice, the molten metal poured into my molds,remains at high temperature less than five minutes for each pouring, noappreciable damage to my mold will take place on repeated heating,except for the normal wear.

For comparison, a block of foamed silica was inserted into an oven at2600 F. and left there for a period of 15 minutes. When removed, thesurface of the foamed silica showed marked signs of disintegration intoa powdery crystalline form. X-ray analysis confirmed that crystals ofsilica were present in the powdered surface.

For a most severe test of my mold, I poured molten metal at atemperature of 2700 F. into a mold I had prepared by using 100 partsamorphous silica to 25 parts binder. Immediately after pouring the metalinto the mold, I immersed the mold in water. Thereafter, the casting wasremoved and the mold inspected. The inspection revealed no damage ordeterioration to the mold.

In still another test, I poured molten metal into a mold made inaccordance with my invention for eighty success'ive pourings, coolingrapidly in air after each pouring and removing the casting thus formed.An inspection of my mold after the eightieth pouring revealed noapparent deterioration to the mold.

From the foregoing discussion it is apparent that I have provided apermanent type mold which incorporates such desirable characteristics asbeing inexpensive, durable and resistant to thermal shock. I have foundthat, parallel to the parting line, tolerances maybe maintained in mymold to .002 inch per linear foot; and, across the parting line, thetolerances may be maintained to Within .004 inch per linear foot. In mycomposite type mold, tolerances may be maintained to the above values ifthe disposable part of the mold is made of amorphous silica. If bondedsand forms the disposable part of my mold, an additional .002 inch perlinear foot should be added to the above toleranees.

In operation, the hot metal may be poured directly against the surfaceof my mold and/or various prior art lubricants, coatings, washes or theabove described shell liners may be used to protect the cavity surfaceof my molds to prevent sticking of the casting in the molds. The linershell, such as shell 32, is normally made so that it will drop out witheach casting.

Since my molds are dry, as compared to the wet green sand, the moltenmetal may be poured at a lower temperature than has, heretofore, beenused for sand castings. Also, my mold may be preheated if desired sothat the surface of the molten metal is not chilled when it strikes thecasting cavity. On the other hand, my molds may be quenched in water orotherwise quickly cooled after receiving the molten metal.

It will be obvious to those skilled in the art that many variations maybe made in the embodiment chosen for purpose of illustration Withoutdeparting from the scope of my invention as defined by the appendedclaims.

I claim:

1. Process of forming a composite mold segment comprising the steps offorming a dispersion of colloidal silica and liquid, commingling finelydivided amorphous silica with said dispersion to form a mix, shapingsaid mix into predetermined shape having an inner surface to define avolume larger than a portion of a casting cavity, forming holes throughsaid shaped mix, removing the liquid from said mix to provide an outershell, fitting said outer shell on a pattern, substantially the size ofsaid portion of a casting cavity and introducing foundry sand throughsaid holes to fill the space between said pattern and said inner surfaceof the shell.

2. The process defined in claim 1 wherein at least one additionalcomposite mold segment is formed and said composite mold segments arefitted together to define a casting cavity surrounded by the foundrysand of said composite mold segments.

References Cited in the file of this patent UNITED STATES PATENTS1,901,427 Alley Mar. 14, 1933 1,918,089 Durand July 11, 1933 2,517,902Luebkeman Aug. 8, 1950 2,521,839 Feagin Sept. 12, 1950 2,795,022 NoelShaw June 11, 1957 FOREIGN PATENTS 575,734 Great Britain Mar. 4, 1946606,119 Great Britain Aug. 6, 1948 1,039,893 France May 20, 1953 203,919Australia Aug. 31, 1956

